The effects of prostaglandins are mediated by their G protein-coupled receptors which are located on the cell surface. Prostaglandin E2 (PGE2) is of particular interest, having a wide variety of cellular effects through binding to functionally different receptor subtypes, namely the EP1, EP2, EP3 and EP4 receptors, all of which respond to PGE2 but differ in their actions.
Dendritic cells (DC) are the most potent antigen-presenting cells of the immune system. Cytokine production by mature antigen-carrying DC within lymph nodes is strongly influenced by PGE2 during their activation in peripheral tissues. Inflammatory cytokines such as IL-1β and TNF-α activate antigen-carrying DC to secrete IL-12 and promote the development of T-helper type 1 (Th-1) cytokine expression-biased cells. In contrast, DC activated in the presence of PGE2 show impaired IL-12 production and promote the development of T-helper type 2 (Th-2) cytokine expression-biased cells [Hilkens C M et al., J. Immunol. 156:1722-27 (1996)]. The difference in the ability to produce IL-12 in response to PGE2, established during DC activation in the peripheral tissues, is stable to the removal of cytokines and PGE2.
Increased production of cytokines triggers inflammation, a normal response by the body to help fight a virus. However, when cytokine production becomes prolonged or excessive it can inflame airways, making it hard to breathe, which in turn can result in pneumonia and acute respiratory distress; and it can injure other organs, which can result in severe life-threatening complications.
It has recently been demonstrated that influenza A subtype H5N1 viruses associated with the recent outbreaks of avian flu in Asia are more potent inducers of inflammatory cytokines and chemokines in primary human alveolar and bronchial epithelial cells in vitro in comparison to the more common, less virulent human flu virus H1N1. Levels of cytokines and chemokines were from 3 times to more than 10 times higher in the human cells infected with the H5N1 virus than those infected with H1N1 (N C W Chan, et al. Respiratory Research 2005, 6:135; article URL: http://respiratory-research.com/content/6/1/135).
These test data correlate with the high levels of cytokines and chemokines seen in patients afflicted with the avian flu, indicating that the hyper-induction of cytokines and/or chemokines is likely relevant to the pathogenesis of human H5N1 disease. Standard steroid anti-inflammatory therapy against avian flu has been of little therapeutic value. Tamiflu® has shown efficacy in that mice infected with H5N1 influenza virus survived when treated. For cases of human infection with H5N1, Tamiflu® may improve prospects for survival but clinical data are limited. Concerns have been recently raised about the safety of Tamiflu® treatment to patients having the avian flu.
It would therefore be desirable to have a therapeutic agent that inhibits the release of overstimulated cytokines and chemokines, especially TNFα interferon gamma (IFN-γ) and Interferon gamma. It would also be desirable to have a therapeutic agent that would treat diseases associated with human H5N1 and other influenza A subtype viruses while being well-tolerated by the patients.